1. Technical Field
The present invention relates to an image display device wherein images are repeatedly displayed and deleted by allowing charged particles to migrate in an electric field.
2. Description of Related Art
Heretofore, as a display device substitutable for a liquid crystal display (LCD), new display devices employing technologies such as an electrophoretic method, an electrochromic method, a thermal method, and a dichroic-particles-rotary method have been proposed. Since these technologies have the advantages described below, the applications thereof to the next generation image display devices, such as a display element for use as a personal digital assistance, or an electronic paper, have received attention. That is, the advantages include: excellent image quality close to regular printed matter since a wider viewing angle can be attained compared to a liquid crystal display, lower electrical power consumption, and the so-called “memory performance”, which is a characteristic of preserving images after the power is off.
Of these technologies, the image display technology employing the electrophoretic method is such that image display is achieved by disposing a solution comprising dispersed particles in a colored solution between a pair of substrates facing each other, followed by applying voltage of about several tens of volts between the substrates, to result in migration of particles in the liquid phase. As an image display technology employing the electrophoretic method, there have been proposed technologies such that a dispersed solution is contained in microcapsules, which are then disposed between a pair of substrates facing each other (for example, refer to Non-Patent Document 1). This technology is considered to be the most feasible for a practical application. However, it has the problem that maintaining a displayed image circumstances is hard to realize.
Specifically, the problem is the difference in the specific gravity between a colored solution and dispersed particles, that is, when the difference in specific gravity between them becomes excessive, the dispersed particles tend to sediment in the colored solution, resulting in a problem of achieving stable image display. For example, when dispersed particles of a high specific gravity, such as titanium oxide, are used in a colored solution of a low specific gravity, the dispersed particles sediment in the colored solution. Further, the colored solution is usually prepared using dyes which are considered to have drawbacks in preservation properties, so that within one aspect of this technology, it is difficult to maintain a constant level of quality of the displayed image.
On the other hand, an image display technology without use of solutions has been proposed. For example, there exists an image display technology in which charged particles are enclosed in a gas phase, and, by applying voltage, the aforesaid particles are allowed to migrate along a direction of an electric field. Although the aforesaid method had no problem such as sedimentation of particles or inferior preservation properties of colored solution, the aforesaid particles were required to be charged by applying voltage between substrates, and moreover, the charged particles were required to migrate along a direction of an electric field which was formed by the applied voltage. In other words, a technology has been desired which creates such a circumstance between substrates that, even under low applied voltage, particles can be smoothly charged and enabled to migrate. In order to respond to the demands, the so-called driving voltage reducing technology was studied, whereby particles are charged and allowed to migrate under low applied voltage, and then, a technology trying to achieve the reduced driving voltage by coating a fluororesin on the surfaces of substrates with which particles come into contact was studied (for example, refer to Patent Document 1).
However, when an image was displayed, charged particles easily attached to the surface of the above-described substrate whose surface was coated by a fluororesin, and to remove the attached particles, high voltage was required to be applied between the substrates. Further, once the substrate surface was attached by the aforesaid particles, displayed image density decreased and image contrast was lowered, to result in deteriorated image quality. As such, the image display device, whose substrate surface was coated with fluororesin, was hard to perform a stable image display under low applied voltage, and in particular, the device showed a tendency for migration of particles between substrates which became increasingly difficult during repeating use.
Patent Document 1: Japanese Unexamined Patent Application Publication No 2003-248247
Non-Patent Document 1: Gugrae-Jo, et al., “New Toner Display Device (I)”, The Annual Conference of the Imaging Society of Japan, Jul. 21, 1999, “Japan Hard Copy '99”, pp. 249-252